This invention relates to a fiber reinforced structural material. More specifically, this invention relates to a composite structure comprised of high strength alumina fibers in a glass matrix.
Fiber reinforced organic matrix composites are widely used and accepted as structural materials because of their desirable attributes of high strength, high modulii and low density. In general, most of these composites comprise an organic polymer matrix, such as an epoxy resin, a polyimide, a polycarbonate, or similar material. The matrices are reinforced with a wide variety of fibers including glass, carbon, graphite and boron. However, even the best of these composites are limited to an operational temperature below about 300.degree. F. (150.degree. C.).
The severe environment encountered by advanced missile systems precludes the use of organic matrices. Radomes for such systems must have acceptable resistance to rain and particle erosion as well as high thermal stability and thermal shock resistance. Generally, ceramic materials meet one or more of these requirements. One further requirement for radomes, that being transparency to X band radiation, precludes the use of certain ceramic materials. Silicon carbide yarn reinforced glass and glass ceramic composites, although very strong, tough, and environmentally stable, have been found to be essentially opaque to X band radiation. Other materials, such as boron nitride reinforced glass and glass ceramic composites, and X band transparent, but are extremely weak and brittle.
In general, the problem of developing tough ceramic fiber-glass or -glass ceramic matrix composites lies with bonding between the fibers and the matrix. In conventional resin matrix composites, such as glass fiber reinforced polyester and carbon fiber reinforced epoxy, toughness is provided by the ability of the system to divert advancing cracks into the fiber-matrix interface, resulting in debonding of fibers and matrix, thus providing an additional energy absorption mechanism by fiber pull-out in the wake of an advancing crack. This results in the so-called "brushy" appearance of the fracture surface of a typical fiber reinforced composite.
In many, if not most ceramic fiber reinforced glass and ceramic matrix composites, bonding between the fibers and the matrices is too strong to permit debonding and fiber pull-out. Consequently, advancing cracks propagate from the matrix into and across the fibers with little or no diversion, thus resulting in a brittle type of fracture.
The problem of defeating too strong a bond formation may, in some instances, be addressed by the application of coatings or films to the fibers which do not bond well to the matrix. The types of materials which are effective in at least partially debonding the ceramic fibers from the matrix material are electrically conductive, which degrade the dielectric properties of the composite.
Thus, what is desired is a composite material which exhibits superior strength and toughness, high thermal stability and is transparent to X band radiation.
Accordingly, it is an object of the present invention to provide an improved ceramic fiber, glass matrix composite material.
It is another object of the invention to provide a method for fabricating an improved ceramic fiber glass matrix composite material.
Other objects and advantages of the present invention will be readily apparent upon consideration of the following detailed description of the invention.